Graphite has been extensively used as thermal diffusion/heat dissipation materials, heat resistant sealing materials, gaskets, separators for fuel cells and the like due to having excellent heat resistance, chemical resistance, thermally conductive properties and electrically conductive properties, and low permeability of gasses. Graphite has thermal and electrical properties that greatly differ between those along the a-b plane direction and the c-axis direction. More specifically, anisotropy of the thermal conductivity in the directions along the a-b plane and the c-axis reaches 50 to 400 times. Heat dissipation films of graphite are employed for allowing thus generated heat to be diffused quickly over a large area taking advantage of such properties. Exemplary methods for producing graphite used in applications of heat dissipation include the following two methods.
One is generally referred to as an expanded graphite method. In this method, expanded graphite, which is yielded when an intercalation compound is formed by treating natural graphite lead with a strong acid such as sulfuric acid and subjected to heating and expansion, is rolled to obtain a graphite film in the form of a sheet (hereinafter, the graphite film produced by this method will be herein referred to as “expanded graphite film”) (Nonpatent Document 1).
Such an expanded graphite film exhibits a thermal conductivity of about 100 to 400 W/(m·K) in directions along the plane, and used as a heat dissipation material. Such an expanded graphite film used for a heat dissipation material is advantageous in ease in production of a sheet having a large area; however, to the contrary, it is disadvantageous in that to achieve a thermal conductivity of 400 W/(m·K) or more is difficult and to produce a thin film having a thickness of 50 μm or less is difficult.
Another method is a polymer thermal decomposition method in which a film of a polymer such as polyoxadiazole, polybenzothiazole, polybenzobisthiazole, polybenzooxazole, polybenzobisoxazole, polythiazole, polyimide, polyphenylenevinylene, or polyimide is subjected to a heat treatment in an inert atmosphere such as argon or helium, or under vacuum (Patent Documents 1, 2 and 3). According to this method, a graphite film is obtained via two steps of: a carbonization step of preheating the polymer film in, e.g., an inert gas, preferably a nitrogen gas at about 1,000° C. to prepare a glassy carbonaceous film; and a graphitization step of thereafter subjecting thus prepared carbonaceous film to a treatment at a temperature of 2,400° C. or more. When viewed as a heat dissipation material, the polymer graphite film is advantageous in exhibiting extremely high thermal conductivity of 600 to 1,800 W/(m·K), being capable of providing a thin sheet and also capable of easily producing a sheet having a thickness of 25 μm or less, whereas it is disadvantageous in difficulty of producing a sheet having a large area.
The following two methods have been known as exemplary method for producing a graphite film by way of the polymer thermal decomposition method:
(Method 1) a method in which a sheet of a material film is sandwiched between graphite plates and subjected to a heat treatment; and
(Method 2) a method in which a long material film is wrapped around a circular cylinder and subjected to a heat treatment.
These methods are explained in more detail as in the following.
(Method 1) Method in which a Sheet of a Material Film is Sandwiched between Graphite Plates and Subjected to a Heat Treatment
Examples 1 and 2 of Patent Documents 1 and 2 disclose a method in which a material film in the form of a sheet is subjected to a heat treatment as follows. A film of PA (poly(m-phenyleneisophthalamide)), PI (poly(pyromellitic imide)), PBI (poly(m-phenylenebenzoimidazole)) or PBBI (poly(m-phenylenebenzobisimidazole)) having a thickness of 25 micron is fixed in a stainless frame, and subjected to a preliminary heat treatment using an electric furnace from room temperature to 700° C. at a rate of 10° C. per minute in argon. Since the film of PA shrinks to 50% of the Original dimension in this temperature range when there is no stainless frame provided, fixation with a stainless frame means that preheating treatment was carried out while applying a tension, as a consequence. The film preheated in this manner is sandwiched with graphite plates, followed by elevating the temperature thereof at a rate of 10° C. per minute in an argon stream, and subjected to a heat treatment at a desired temperature (Tp) for 1 hour. Subsequently, the film is cooled after the heat treatment at a rate of temperature fall of 20° C. per minute. The furnace used is an electric furnace in which a carbon heater is employed. Thus obtained black film is fragile and does not have flexibility when the Tp is 1,400° C. or less; however, it has flexibility when the Tp is 1,800° C. or more.
(Method 2) Method in which a Long Material Film is Wrapped Around a Circular Cylinder and Subjected to a Heat Treatment
Example 1 of Patent Document 3 discloses a method in which a long material film is wrapped around a circular cylinder and subjected to a heat treatment as in the following. A POD film having a width of 180 mm and a thickness of 50 μm is triply wrapped around a graphitous carbon circular cylinder having an external diameter of 68 mm, an internal diameter of 64 mm and a length of 200 mm (i.e., three sheets being overlaid, not one sheet being wrapped around three times), followed by elevating the temperature thereof from room temperature at a rate of 10° C. per minute in an argon stream, and subjected to a treatment at a desired temperature Tp for 1 hour. Thereafter, the temperature is fallen at a rate of 20° C. per minute. The furnace employed may be a 46-6 type carbon heater furnace manufactured by SHINSEI DENRO Ltd. Thus obtained black film is fragile and does not have flexibility when the heat treatment temperature Tp is 1,600° C. or less; however, it has flexibility when the Tp is 1,800° C. or more. The film has a size of 170×180 mm. In addition, a method in which a film that serves as a separator is wrapped around a circular cylinder together with a material film, and a heat treatment is concurrently carried out, for preventing the material film from fusion with one another is proposed in Patent Document 4. Furthermore, Patent Document 4 discloses in the section of Problems to be Solved by the Invention that obtaining a graphite film having a length not shorter than the cylindrical circumference is difficult unless a separator is used.